Shower drain always slow, what can I do?

It's not always an easy problem to solve because it depends on where the clog is building up. If it's just the showerhead, usually it's a five-minute fix. If the blockage is deeper in the pipes, then it gets more complicated.

I'd start with the showerhead: take it apart and check the little holes. Usually that's where limescale and soap residue accumulate. Soak it for an hour in a bowl with white vinegar (or vinegar and lukewarm water), then scrub away the dirt with a toothbrush. This fixes the problem in about 70% of cases, at least from what I see at home. If the showerhead isn't the culprit, the blockage is in the main drain line or in the trap under the shower pan. Then you can try a plunger or a drain vacuum for showers, which are pretty effective. If that doesn't work either - like the shower just won't drain at all - at that point you need a plumber, because it could be a serious obstruction inside the pipes or the pipes might be pitched wrong so the water can't flow properly.

In the meantime, don't throw stuff down there - hair, whole bars of soap - and if you have areas with a lot of limescale, rinse the showerhead in vinegar every week. Little maintenance like that keeps everything flowing.